Can anyone tell me what size the 1.9L m44 throttle body is, and will a larger throttle body from another BMW bolt up? If not I would like to know the size so I can get mine bored out and use a plate from another vehicle.

The stock larger plate size is 52mm, the smaller plate I have no idea. No other throttle bodies from any other BMW will fit the M44. M42/M44 is the only TB which is dual stage (consists of 2 throttle body plates, not 1 like regular ones). There's no point in swapping throttle bodies. Just bore out your current one and have a larger butterfly plate installed. Universal butterflies will NOT work. You'll have to get one custom made.

I suggest 52 => 54mm or 55.5mm (so +2 or +3.5)

I had mine increase 3.5mm larger. Leave the smaller plate alone. It's good to have that plate there for some low end torque.